Text-to-speech programs allow you to type in text and create a spoken word audio sample. They usually have different 'voices' to choose from, and allow you to alter the speed and pitch the words are spoken at.
Here's a Google search for text-to-speech synthesiser.
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Programs
Two good ones are AT&T Labs Natural Voices and Cepstral Text-to-Speech.
You can use their online demo's which both allow you to save a wav file to disk.
Rivaling Microsoft Sam, Macs also have a built in text to speech program. (I'm unsure what it is called...PC user here.)
FL Studio has a text-to-speech plugin built right in! ---> http://flstudio.image-line.com/help/htm ... 0Synth.htm

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Techniques
You may find it takes some tweaking to get the voices sounding right. Try editing the pitch and speed of the voice, and spelling words how they sound. Unusual placement of punctuation can also alter the tone and inflection of the voices, so try placing commas and full stops in different places to see if it improves the clarity of the pronunciation.
Entering one word at a time can often yield more natural results. with each word as its own sample, you can use your sequencer to adjust the timing between words more asily.
Many effects can be used to make the results of a text-to-speech synthesiser sound better. These include using chorussing, reverb, pitchshifting, timestretching and compression.
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Text-to-Speech vs Real Voice Actors
If you have a microphone, it will sound a lot more natural if you can record real people.
Writing a script before recording often helps actors to deliver better results. Try roping your mates and family into being voice actors for you.
